Ukraine reinforced its defenses at the port of Mariupol on the Sea of Azov on Thursday in anticipation of a rebel attack amid reports that separatists were advancing on the city with tanks and artillery.

Mariupol is Ukraine's largest port on the Sea of Azov and stands on the way to Russian-annexed Crimea. It has become a focus of Ukrainian concern after rebels broke away from their main strongholds further north and took the nearby coastal town of Novoazovsk, to the east of Mariupol, in late August.

A Ukrainian soldier told Reuters he had seen pro-Russian separatists advancing on the city with tanks, armored personnel carriers and artillery. His comments could not be independently confirmed.

Separately, a Ukrainian officer said the rebels had been taking over army checkpoints about seven km (five miles) away, adding: "There will be an attack, we are waiting for them here (in Mariupol)."

Kostyantyn Batozsky, deputy governor of the Donetsk region where Mariupol is located, said on his Facebook page that Ukrainian forces had destroyed four tanks belonging to advancing pro-Russian separatists.

A Reuters witness heard about a dozen blasts and saw plumes of black smoke rising a few km (miles) to the east of Mariupol. He also saw an armored personnel carrier conveying a wounded soldier into the city.

The governor of Donetsk region, Serhiy Taruta, vowed to defend the city from the rebels during a visit to frontline troops but also said he hoped tentative plans for a ceasefire in the region on Friday would be confirmed.

"We are fighting to repel the DNR (the rebels' self-proclaimed Donetsk People's Republic), Russia and whoever else wants to come here. We are defending the city," he said.

"We are hoping for a ceasefire, talks and resolution of all unresolved issues within a sovereign Ukraine," Taruta added.

Earlier, while attending a NATO summit in Wales, Ukraine's President Petro Poroshenko said he expected envoys due to meet in Minsk on Friday to agree a ceasefire that would pave the way for implementation of a peace plan for his country.